Pattern comparison ultrasonic surveillance system with noise suppression

ABSTRACT

An ultrasonic surveillance system which emits pulse-like ultrasonic waves to an area under surveillance, which statistically compares a reflected-wave pattern established by receiving ultrasonic waves reflected from the under surveillance with a predetermined reference pattern, and which produces an alarm signal when these two patterns are not in agreement. According to this system, the alarm signal is prevented from being erroneously produced by ultrasonic noises.

DESCRIPTION

1. Technical Field

The present invention relates to an ultrasonic surveillance system and,more particularly, to an ultrasonic surveillance system utilizingpulse-like ultrasonic waves.

2. Background Art

Ultrasonic waves have often been utilized for modern surveillancesystems which produce alarms when an unauthorized person enters intopredetermined areas. There has heretofore been proposed an ultrasonicsurveillance system of this type utilizing the Doppler effect. Accordingto this system, ultrasonic waves of a predetermined frequency arecontinuously emitted into an area under surveillance which may be in aroom, and when frequency components different from the predeterminedfrequency are detected being superposed on the waves which are reflectedby the objects in the area under surveillance, the presence of saidunauthorized person in the area under surveillance is detected, and thealarm, such as a buzzer, is energized. Such a system utilizes theDoppler effect, according to which the frequency of the reflected wavesincreases when an object approaches the ultrasonic wave emitter, and thefrequency of the reflected waves decreases when an object moves awayfrom the ultrasonic wave emitter. In order for the alarm not to operateeven when ultrasonic waves (hereinafter referred to as ultrasonic noise)generated by a telephone bell or a bell of fire alarm have infiltratedinto the reflected waves, in other words, in order to prevent the alarmfrom being erroneously operated by ultrasonic noise, the above-mentionedsystem has been so designed that the received signals are detected in afrequency-modulated manner and are allowed to pass through a low-passfilter to remove the frequency components caused by the ultrasonicnoise, since the frequency components (period in the change offrequency) of the ultrasonic noise are much greater than the frequencycomponents of the reflected waves.

There has also been proposed an ultrasonic surveillance system utilizingpulse-like ultrasonic waves. According to this system, pulse-likeultrasonic waves are emitted to an area under surveillance, wavesreflected by the objects in the area under surveillance are received tocompare the pattern of the reflected waves with a predeterminedreference pattern statistically, and an alarm is produced when the twopatterns are not in agreement. With this system, however, the ultrasonicnoises are not removed even when the received signals are allowed topass through the low-pass filter, since the frequency components ofultrasonic noises are superposed on the frequency components ofreflected waves. Therefore, when the ultrasonic noises infiltrate intothe reflected waves, the pattern of the reflected waves is changed toerroneously produce the alarm. Accordingly, in a room where ultrasonicnoises are likely to be produced, a limitation is imposed on the placefor installing such a surveillance device. Further, even if it isinstalled at a position which is considered to be acceptable, thesurveillance device may erroneously operate, thus decreasing thereliability of the device.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide an ultrasonicsurveillance system having high reliability, which will not beerroneously operated by ultrasonic noises that are generated by atelephone bell or by a bell of a fire alarm.

According to the present invention, there is provided an ultrasonicsurveillance system comprising a transmitter means which transmitspulse-like ultrasonic waves to an area under surveillance, receivingmeans which receives ultrasonic waves reflected from the area undersurveillance and which subjects the signals of ultrasonic waves toanalog-to-digital conversion, and a first operation processing meanswhich compares a pattern of reflected waves established by the digitalsignals sent from the receiving means with a reference patternstatistically and which produces an alarm signal when the two patternsare not in agreement, wherein the improvement is characterized by theprovision of means for preventing erroneous operation by preventing thealarm signal from being erroneously produced by the ultrasonic noiseswhich are generated in the area under surveillance.

In the ultrasonic surveillance system according to the presentinvention, ultrasonic noise of a relatively small amplitude is offset bythe difference between the output of a first frequency selection meansand the output of a second frequency selection means. Ultrasonic noiseof a relatively large amplitude is detected by integrating the output ofthe second frequency selection means; therefore, the analog-to-digitalconversion operation of a receiver circuit is stopped so that an alarmsignal is not generated. Furthermore, ultrasonic noise having a shortperiod of variation is detected by counting peak values (maximum valuesor minimum values) in the output of the first frequency selection means,thereby to inhibit the production of an alarm signal. Thus, generationof alarm signals by ultrasonic noises is minimized to prevent erroneousoperation of the alarm.

The invention will become more apparent from the following descriptionwith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B comprise a block diagram illustrating a generalultrasonic surveillance system utilizing pulse-like ultrasonic waves;

FIGS. 2A through 2D are diagrams illustrating timings of signals whichappear in the circuit of FIG. 1;

FIGS. 3A and 3B comprise a block diagram illustrating a first embodimentof the ultrasonic surveillance system according to the presentinvention;

FIGS. 4A through 4E, 5A through 5E, 6A through 6E, and 7A through 7E aretime diagrams of signals which appear in the circuit of FIG. 3;

FIGS. 8A and 8B comprise a block diagram illustrating a secondembodiment of the ultrasonic surveillance system according to thepresent invention;

FIGS. 9A through 9E are time diagrams of signals which appear in thecircuit of FIG. 8; and

FIGS. 10A and 10B comprise a block diagram illustrating a thirdembodiment of the ultrasonic surveillance system according to thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, which illustrates a general ultrasonic surveillancesystem utilizing pulse-like ultrasonic waves, the transmitting operationof an ultrasonic transmitter-receiver element (hereinafter referred toas ultrasonic element) 1 is controlled by a transmitter circuit 2.Ultrasonic waves emitted from the ultrasonic element 1 are reflected bydesks, shelves and walls in an area under surveillance, and are receivedby the ultrasonic element 1. The reflected waves which are received areconverted into digital signals by a receiver circuit 3, and aretransmitted to an operation processing circuit 4 where a referencepattern is prepared based upon a pattern of reflected waves collected bythe receiver circuit 3, and the pattern of reflected waves correspondingto the conditions in the area under surveillance is statisticallycompared with the reference pattern when the area is under surveillance.When the two patterns are not in agreement, the operation processingcircuit 4 produces alarm signals to energize an alarm 5 such as buzzer.

FIGS. 2A through 2D are time diagrams of signals which are produced inthe circuit of FIG. 1. The circuit of FIG. 1 will now be described belowwith reference to FIGS. 2A through 2D. In the transmitter circuit 2, anoscillator 21 produces signals "a" of a frequency of 25 through 50 kHz,as illustrated in FIG. 2A, and a pulse oscillator 22 produces pulse-likesignals "b" having a repetitive period T of the order of, for example,100 milliseconds and a duration time of the order of severalmilliseconds, as illustrated in FIG. 2B. A modulator 23, therefore,produces signals "c" as illustrated in FIG. 2C. The signals emitted fromthe ultrasonic element 1, responsive to the signals "c", are reflectedby the desks, shelves and walls in the area under surveillance, andthese many reflected waves are received by the ultrasonic element 1 in asuperposed manner, and are converted into electric signals "d".

In the receiver circuit 3, the signal "d" is amplified by an amplifier31, rectified by a rectifier 32 for amplitude-modulated detection,smoothed by a smoothing circuit 33, and is converted into analog signals"d₁ " as illustrated in FIG. 2D. The analog signals "d₁ " are sampled inan analog-to-digital converter 34 by clock pulses, with maintaining apredetermined time interval, generated from a clock oscillator 35, andare converted into digital signals "e".

In order to obtain a reference pattern based upon the reflected wavesspecific to the area under surveillance where there is no personpresent, a counter 24 in the transmitter circuit 2 is reset by a resetsignal "R". The counter 24 counts the pulses generated by the pulseoscillator 22, and a comparator 25 compares the value of the counter 24with a predetermined value N. When the value of the counter 24 issmaller than the predetermined value N, the comparator produces a signalof a high potential and when the value of the counter 24 is greater thanthe predetermined value N, the comparator 25 produces a signal of a lowpotential. Therefore, gates 41-1 and 41-2 of the operation processingcircuit 4 are, respectively, opened and closed with respect to thedigital signals "e" for a predetermined period of time after the counter24 has been reset. Accordingly, the data of digital signals "e" isstored via the gate 41-1 in a buffer memory 42 at an address specifiedby an address generator 43. The address generator 43 is controlled bythe pulse oscillator 22 of the transmitter circuit 2 and by the clockoscillator of the receiver circuit 3. Thus, the buffer memory 42 storesa plurality of reflected-wave patterns of a predetermined number ofsamples as reference patterns. The operation for storing a pattern iscompleted when the value of the counter 24 in the transmitter circuit 2has reached a predetermined value. Thereafter, mean values of theconstants stored in the buffer memory 42 are calculated by a mean valuecalculator circuit 44 for each sampling point, and the calculatedresults are stored in a memory 45. At the same time, standard deviationsin the contents stored in the buffer memory 42 are calculated bystandard deviation calculator circuit 46 for each sampling point, andthe calculated results are stored in memory 47.

Next, since the output signal of the comparator 25 assumes a lowpotential, the gates 41-1 and 41-2 of the operation processing circuit 4are, respectively, closed and opened for the digital signals "e".Consequently, the digital signals "e" are supplied to an input ofdifference computing circuit 48 via the gate 41-2, while another inputof the difference computing circuit 48 is supplied with mean values foreach sampling point from the memory 45. Therefore, the differencebetween the reflected-wave pattern from the receiver circuit 3 and themean value of the reference pattern stored in the memory 45 iscalculated for every sampling point. A discrimination circuit 49compares the output of the difference computing circuit 48 with the dataof standard deviation stored in the memory 47 for every sampling point.When the output of the circuit 48 is greater than the standarddeviation, the discrimination circuit 49 produces an alarm signal toenergize the alarm 5 such as a buzzer. As mentioned above, thereflected-wave pattern is statistically compared with the referencepattern which is stored in the buffer memory 42, and the alarm 5 isenergized when the two patterns are not in agreement. Referring to FIG.2D in which a solid line represents the reference pattern, the alarm 5will be energized when, for example, a portion indicated by an arrow Xemerges in the reflected-wave pattern due to the presence of a personwho has entered the area under surveillance. The above-mentiond circuit,however, presents a problem in that the alarm 5 will be energized evenwhen a portion indicated by an arrow Y appears in the reflected-wavepattern being caused by ultrasonic noise from a ringing bell of atelephone or a fire alarm.

The present invention was accomplished based upon the discovery thatultrasonic noise exhibits the following three phenomena.

I. Ultrasnoic noise having a relatively small amplitude such as of aringing bell of a telephone is contained in the frequency band ofultrasonic waves emitted by the ultrasonic element and is also containedin other frequency bands.

II. Ultrasonic noise having a relatively large amplitude such as of aringing bell of a fire alarm is mostly contained in frequency bandsother than the frequency band of ultrasonic waves emitted by theultrasonic element.

III. Ultrasonic noise appears in a reflected-wave pattern in the form ofa wave of a relatively small period.

According to the present invention relying upon the phenomenon I, thereflected-wave pattern is formed based upon a difference between apattern of frequency components of the emitted ultrasonic waves and apattern of frequency components other than the above-mentioned frequencycomponents, thereby to remove ultrasonic noise having relatively a smallamplitude from the reflected-wave pattern. Therefore, the alarm can beprevented from being erroneously operated by an ultrasonic noise ofrelatively small amplitude. Utilizing the phenomenon II, furthermore,the alarm 5 can be prevented from being erroneously operated by anultrasonic noise of relatively large amplitude by integrating thefrequency components other than those of the emitted ultrasonic wavesfor a predetermined period of time, and stopping the analog-to-digitalconversion of the reflected-wave pattern when the value of integrationis greater than a predetermined value. Relying upon the phenomenon III,moreover, the number of maximum or minimum values in the reflected-wavepattern is counted to statistically compare the counted value with areference value of a number of said values in the reference pattern. Analarm signal is not produced when the number of said values is greaterthan the reference value, thereby to prevent the alarm from beingerroneously operated by the ultrasonic noise.

FIG. 3 is a block diagram illustrating a first embodiment of theultrasonic surveillance system according to the present invention, inwhich the elements which are the same as those of FIG. 1 are denoted bythe same reference numerals as those of FIG. 1. A receiver circuit 3' isdifferent from the receiver circuit 3 of FIG. 1. In the receiver circuit3', a band-pass filter 36-1 which serves as a first frequency selectionmeans to permit the passage of frequency components of the emittedultrasonic waves and a band-pass filter 36-2 which serves as a secondfrequency selection means to permit the passage of frequency componentsother than those of the emitted ultrasonic waves, are connected to theoutput of the amplifier 31. Output signals of the two band-pass filters36-1 and 36-2 are rectified by rectifiers 32-1 and 32-2 foramplitude-modulated detection. A difference between an output signal "f"and an output signal "g" of the rectifiers 32-1 and 32-2, respectively,is detected by a differential amplifier 37, and an output signal "h" ofthe differential amplifier 37 is supplied to the smoothing circuit 33.

When an ultrasonic noise of a relatively small amplitude, such as of theringing bell of a telephone is present in the area under surveillance,the wave-forms of the ultrasonic noise appear both in the band-passfilter 36-1 and in the band-pass filter 36-2. In this case, if thereflected-wave components in the output signal "f" of the rectifier 32-1is denoted by S, the noise components by N₁, and the noise components inthe output signal "g" of the rectifier 32-2 by N₂, the amplitude of theoutput signal "h" of the differential amplifier 37 is proportional toS+(N₁ -N₂). With reference to ultrasonic noises of relatively smallamplitudes, such as of a ringing bell of a telephone, there is anintimate correlation between N₁ and N₂ ; noise components N₁ are set tobe nearly equal to noise components N₂ by adjusting the amplitude andfrequency characteristics and by adjusting the amplification factor ofthe band-pass filters 36-1 and 36-2. Therefore, the ultrasonic noises ofrelatively small amplitudes do not appear in the output of thedifferential amplifier 37.

Referring to the receiver circuit 3', furthermore, an integrator 38 anda comparator 39 are connected in the subsequent stages of the rectifier32-2 to detect ultrasonic noises of relatively large amplitudes such asof a bell of a fire alarm. The amplitude of an output signal "j" of theintegrator 38 increases with the increase in the output of the band-passfilter 36-2. As the amplitude exceeds a reference value V_(R), thepotential of the output signal of the comparator 39 is converted from alow level to a high level, whereby a gate 40 is closed to interrupt theanalog-to-digital converter 34. Therefore, the analog-to-digitalconverter 34 discontinues the operation of analog-to-digital conversion,and the alarm 5 is prevented from being erroneously operated by theultrasonic noises of relatively large amplitudes. Here, the integrator38 is reset by the output signal "b" of the pulse oscillator 22 in thetransmitter circuit 2.

Operation of the circuit of FIG. 3 will now be explained below indetail.

FIGS. 4A through 4E are diagrams illustrating timings of signals whichare produced in the circuit of FIG. 3 when there is no person present inthe area under surveillance and no ultrasonic noise is present. When theultrasonic element 1 emits ultrasonic waves to the area undersurveillance responsive to the modulated signal "c" illustrated in FIG.4A, the rectifier 32-1 in the receiver circuit 3' produces the outputsignal "f" as illustrated in FIG. 4B. Here, since no ultrasonic noise ispresent, the potential of the output signal "g" of the rectifier 32-2 ismaintained at zero as illustrated in FIG. 4C. As illustrated in FIG. 4D,therefore, the wave-form of an output signal "h" of the differentialamplifier 37 resembles the wave-form of FIG. 4B. Further, since thepotential of the output signal "g" of the rectifier 32-2 is zero, thepotential of the output signal "j" of the integrator 38 is maintained atzero as illustrated in FIG. 4E. When the differential amplifier 37produces the output signal "h" of the wave-form as illustrated in FIG.4D, the operation processing circuit 4 determines that thereflected-wave pattern is the same as the reference pattern, and thealarm 5 is not energized.

FIGS. 5A through 5E are diagrams illustrating timings of signals whichare produced in the circuit of FIG. 3 when a person is present in thearea under surveillance and when no ultrasonic noise is present. Whenthe ultrasonic element 1 emits ultrasonic waves to the area undersurveillance responsive to the modulated signal "c" illustrated in FIG.5A, the output signal "f" of the rectifier 32-1 in the receiver circuit3' contains the wave X which is reflected by a person who has enteredthe area as illustrated in FIG. 5B. Since no ultrasonic noise ispresent, however, the potential of the output signal "g" of therectifier 32-2 is maintained at zero as illustrated in FIG. 5C. As shownin FIG. 5D, therefore, the wave-form of the output signal "h" of thedifferential amplifier 37 resembles the wave-form of FIG. 5B. Further,since the potential of the output signal "g" of the rectifier 32-2 iszero, the potential of the output signal "j" of the integrator 38 ismaintained at zero as illustrated in FIG. 5E. When the differentialamplifier 37 produces the output signal "h" of the wave-form asillustrated in FIG. 5D, the operation processing circuit 4 determinesthat the reflected-wave pattern (corresponding to FIG. 5D) is differentfrom the reference pattern (corresponding to FIG. 4D), and the alarm 5is energized.

FIGS. 6A through 6E are diagrams illustrating timings of signals whichare produced in the circuit of FIG. 3 when there is no person present inthe area under surveillance but when there is present ultrasonic noiseof relatively small amplitudes such as a ringing of a bell of atelephone. When the ultrasonic element 1 emits ultrasonic waves to thearea under surveillance responseive to the modulated signal "c"illustrated in FIG. 6A, the output signal "f" of the rectifier 32-1 inthe receiver circuit 3' contains the wave Y which is caused by theultrasonic noise as illustrated in FIG. 6B. Further, the output signal"g" of the rectifier 32-2 contains only the wave caused by theultrasonic noise as illustrated in FIG. 6C. The output signal "h" of thedifferential amplifier 37, however, does not contain the wave caused bythe ultrasonic noise as illustrated in FIG. 6D. Namely, the wave-form ofFIG. 6D resembles the wave-form of the reference pattern of FIG. 4D.Moreover, the output signal "g" of the rectifier 32-2 has such a smallamplitude that the potential of the output signal "j" of the integrator38 does not reach the reference value V_(R) as illustrated in FIG. 6E.Accordingly, when the differential amplifier 37 produces the outputsignal "h" of the wave-form as illustrated in FIG. 6D, the operationprocessing circuit 4 determines that the reflected-wave pattern(corresponding to FIG. 6D) is in agreement with the reference pattern(corresponding to FIG. 4D), and the alarm 5 is not energized.

FIGS. 7A through 7E are diagrams illustrating timings of signals whichare produced in the circuit of FIG. 3 when there is no person present inthe area under surveillance but when ultrasonic noise of a relativelylarge amplitude such as of a bell of a fire alarm is present. When theultrasonic element 1 emits ultrasonic waves to the area undersurveillance responsive to the modulated signal "c" which is illustratedin FIG. 7A, the output signal "f" of the rectifier 32-1 in the receivercircuit 3' contains a wave Z of large amplitudes caused by theultrasonic noise as illustrated in FIG. 7B. Further, the output signal"g" of the rectifier 32-2 contains waves of large amplitudes caused bythe ultrasonic noise as illustrated in FIG. 7C. Due to the presence ofthe wave Z of large amplitudes in the output signal "f", the outputsignal "h" of the differential amplifier 37 contains an abnormal wave Z'as illustrated in FIG. 7D, which has the potential to energize the alarm5. The potential of the output signal "j" of the integrator 38, however,is greater than the reference value V_(R) as illustrated in FIG. 7E.Consequently, the comparator 39 produces a signal of a low potential toclose the gate 40; the analog-to-digital conversion operation is ceasedand the alarm 5 is not energized. In other words, the alarm 5 is notoperated even when abnormal waves Z' caused by ultrasonic noise arepresent in the output signal "h" of the differential amplifier 37.

FIG. 8 is a block diagram illustrating a second embodiment of theultrasonic surveillance system according to the present invention, inwhich the elements which are the same as those of FIG. 1 are denoted bythe same reference numerals. A receiver circuit 3" is different from thereceiver circuit 3 of FIG. 1. Referring to the receiver circuit 3", apeak detecting circuit 101 for detecting maximum values or minimumvalues, and a counter 102 are provided responsive to the output of therectifier 32. The peak detecting circuit 101 consists of adifferentiation circuit and a comparator which are connected in series,and the counter 102 is reset by the output signal "b" of the pulseoscillator 22 in the transmitter circuit 2. Referring to FIG. 8B,furthermore, a second operation processing circuit 6 is provided inaddition to the first operation processing circuit 4, to statisticallycompare the number of peak values in the reflected-wave pattern with thenumber of peak values in the reference pattern.

The second operation processing circuit 6 is constructed similarly tothe first operation processing circuit 4. A buffer memory 62 stores thenumber of peak values of plurality of reference patterns. Namely, theoutput signal of the comparator 25 assumes the high potential for apredetermined period of time after the counter 24 in the transmittercircuit 2 has been reset, and gates 61-1 and 61-2 are opened and closed,respectively, for the output signal of the counter 102. Hence, theoutput signals of the counter 24 are stored, via the gate 61-1, in amemory location of the buffer memory 62, the address of which isspecified by an address generator 63. The address generator 63 iscontrolled by the pulse oscillator 22 in the transmitter circuit 2.Thus, the buffer memory 62 stores the number of peak values of apredetermined number of reference patterns. Thereafter, mean values andstandard deviations of the contents stored in the buffer memory 62 arecalculated by a mean value calculator circuit 64 and a standarddeviation calculator circuit 66. The calculated results are stored inmemories 65 and 67.

Then, the potential of the output signal of the comparator 25 changesfrom the high level to the low level, and the gates 61-1 and 61-2 of theoperation processing circuit 6 are closed and opened, respectively, forthe output signal of the counter 102. Hence, the output signal of thecounter 102 is supplied to an input of a difference computing circuit 68via the gate 61-2, while another input of the difference computingcircuit 68 is supplied with a mean value of the number of peak valuesfrom the memory 65. Consequently, the difference computing circuit 68sends the difference between a mean value of the number of peak valuesin the reflected-wave pattern from the receiver circuit 3" and a meanvalue of the number of peak values in the reference pattern stored inthe memory 65, to a discrimination circuit 69. When the above-mentioneddifference is greater than a standard deviation stored in the memory 67,the discrimination circuit 69 produces a signal of the high potential toclose a gate 7. Irrespective of the operation of the first operationprocessing circuit 4, therefore, the alarm signal is not produced, andthe alarm 5 is not energized. According to the second embodiment asmentioned above, the number of peak values in the reflected-wave patternis statistically compared with the number of peak values of a referencepattern stored in the buffer memory 62, to detect the ultrasonic noisecaused by the ringing bell of a telephone or by the bell of a firealarm, in order to prevent the alarm 5 from being erroneously operatedby the ultrasonic noise.

FIGS. 9A through 9E are diagrams illustrating timings of signals whichare produced in the circuit of FIG. 8. FIG. 9A illustrates modulatedsignals "c" for emitting ultrasonic waves from the ultrasonic element 1to the area under surveillance. FIG. 9B illustrates an output signal "u"of the rectifier 32 in the receiver circuit 3" when no person is presentin the area under surveillance and when there is no ultrasonic noise. Inthis case, as illustrated in FIG. 9C, output signals "v" of the peakdetecting circuit 101 produce a train of pulses which correspond to amaximum value of the signal "u" of FIG. 9B. FIG. 9D, on the other hand,illustrates an output signal "u" of the rectifier 32 when there isultrasonic noise in the area under surveillance. Since varying periodsof amplitudes of the ultrasonic noise are short, abnormal portions U, Vand W develop each having a very increased number of maximum points asshown in FIG. 9D. In this case, as illustrated in FIG. 9E, the outputsignal "v" of the peak detecting circuit 101 in the receiver circuit 3"become a series of pulses. The second operation processing circuit 6statistically compares the number of pulses in the pulse series of FIG.9C and FIG. 9E. Therefore, even when the ultrasonic noise, is caused byperson entering the area under surveillance is detected by the firstoperation processing circuit 4, the second operation processing circuit6 keeps the gate 7 closed, and the alarm 5 is not energized.

FIG. 10 is a block diagram illustrating a third embodiment of theultrasonic surveillance system according to the present invention. Thethird embodiment of FIG. 10 is made up of a combination of the firstembodiment of FIG. 3 and the second embodiment of FIG. 8. When awave-form having very many maximum points is contained in the outputsignal "f" of the rectifier 32-1, or when a wave-form having largeamplitudes is contained in the output signal "g" of the rectifier 32-2,a receiver circuit 3"' regards them as those caused by the ultrasonicnoise, so that the alarm 5 is not energized.

In the above embodiments, the contents (reference patterns and referencevalues) stored in the buffer memories 42 and 62 of the operationprocessing circuits 4 and 6 can be rewritten at any time by resettingthe counter 24 in the transmitter circuit 2.

In the ultrasonic surveillance system according to the present inventionas described in the foregoing, the alarm can be prevented from beingerroneously operated by the ultrasonic noise such as of the ringing bellof a telephone or the ringing bell of a fire alarm, and the reliabilityof the device can be enhanced.

I claim:
 1. An ultrasonic surveillance system comprising: transmittermeans for transmitting pulse-like ultrasonic waves to an area undersurveillance; receiving means for receiving ultrasonic waves reflectedfrom the area under surveillance to convert the reflected ultrasonicwaves into electrial signals and for subjecting the electrical signalsto analog-to-digital conversion; and an operation processing means forstatistically comparing a reflection pattern established by the digitalsignals sent from said receiving means with reference patterns basedupon the area under surveillance when there is no obstacle, to producean alarm signal when these patterns are not in agreement, characterizedin that said receiving means comprises:a first frequency means forpermitting frequencies of said pulse-like ultrasonic waves to passtherethrough; a second frequency selection means for permittingfrequencies other than the frequencies of said ultrasonic waves to passtherethrough; an analog-to-digital convertor converting the outputsignal of said first frequency selection means; an integrator forintegrating the output signal of said second frequency selection means;and interrupting means for interrupting the signal transmission fromsaid first frequency selection means to said analog-to-digital converterwhen the output level of said integrator exceeds a predetermined value.2. A system as set forth in claim 1, wherein said operation processingmeans comprises:a buffer memory for storing reference patterns; a meanvalue calculator means for calculating mean values of the data stored insaid buffer memory for every sampling point; a first memory for storingthe calculated means values of said mean value calculating means; astandard deviation calculator means for calculating standard deviationsof the data stored in said buffer memory for every sampling point; asecond memory for storing the calculated standard deviation values ofsaid standard deviation calculator means; a difference computing meansfor calculating the difference between the data sent from saidanalog-to-digital converter and the data stored in said first memory forevery sampling point; and a discrimination means for comparing thecalculated differences of said difference computing means with thestandard deviations stored in said second memory for every samplingpoint to produce an alarm signal when one of the calculated differencesis greater than the corresponding standard deviation.
 3. An ultrasonicsurveillance system comprising: transmitter means for transmittingpulse-like ultrasonic waves to an area under surveillance; receivingmeans for receiving ultrasonic waves reflected from the area undersurveillance to convert the reflected ultrasonic waves into electricalsignals and for subjecting the electrical signals to analog-to-digitalconversion; and a first operation processing means for statisticallycomparing a reflection pattern established by the digital signals sentfrom said receiving means with reference patterns based upon the areaunder surveillance when there is no obstacle, to produce an alarm signalwhen these patterns are not in agreement, characterized in that saidreceiving means comprises:a peak detector for detecting peak values inthe received electrical signals; and a counter for counting the numberof the peak values detected by said peak detector, said system furthercomprising: a second operation processing means for statisticallycomparing the number of peak values produced from said counter of saidreceiving means with a reference value; and an interrupting means forinterrupting the alarm signal of said first operation processing meanswhen the number of peak values exceeds the reference value.
 4. A systemas set forth in claim 3, wherein said first operation processing meanscomprises:a buffer memory for storing reference patterns; a mean valuecalculator means for calculating mean values of the data stored in saidbuffer memory for every sampling point; a first memory for storing thecalculated mean values of said mean value calculating means; a standarddeviation calculator means for calculating standard deviations of thedata stored in said buffer memory for every sampling point; a secondmemory for storing the calculated standard deviation values of saidstandard deviation calculator means; a difference computing means forcalculating the difference between the data sent from saidanalog-to-digital converter and the data stored in said first memory forevery sampling point; and a discrimination means for comparing thecalculated differences of said difference computing means with thestandard deviations stored in said second memory for every samplingpoint to produce an alarm signal when one of the calculated differencesis greater than the corresponding standard deviation.
 5. A system as setforth in claim 3, wherein said second operation processing meanscomprises:a buffer memory for storing the number of peak values of aplurality of reference patterns; a mean value calculator means forcalculating a mean value of the data stored in said buffer memory; afirst memory for storing the calculated mean value of said mean valuecalculating means; a standard deviation calculator means for calculatinga standard deviation of the data stored in said buffer memory; a secondmemory for storing the calculated standard deviation value of saidstandard deviation calculator means; a difference computing means forcalculating the difference between the data sent from said counter andthe data stored in said first memory; and a discrimination means forcomparing the calculated difference of said difference computing meanswith the standard deviation stored in said second memory to produce analarm signal when one of the calculated differences is greater than thecorresponding standard deviation.